One of the final achievements of a military-oriented society is the introduction of a professional army. Unlike spontaneously created armies which rely on wartime conscription of civilians, professional soldiers dedicate their entire lives to training and preparation for war. And when it comes, they are prepared for it much better than conscripts. But the best benefits appear exactly in the preparatory part: all soldiers are dedicated to constantly upgrading their training structures and equipment, thus reducing the resources your empire needs to spend for these activities!

This is the high-end Policy of the Honor tree. It is not very useful in the beginning of the game, when you haven't yet developed techs which require your units to upgrade, and when you usually have priorities other than constructing military buildings in your cities. In the late game the Policy becomes especially useful, since the Gold cost of some upgrades becomes exorbitant.

A professional army is a full-time military organization that remains in being even when the country is at peace. Its soldiers are paid for their services by the state and spend their time training or performing public works when not at war. The earliest armies were not professional. When war was imminent, the country's leaders would issue a call and all able-bodied men would shoulder their weapons and march off to battle. These amateur soldiers usually didn't have much training and were often expected to supply their own weapons and armor, so their quality was not very high. They tended to desert when they had to tend to their crops, so battles were scheduled for the summer. If victory wasn't achieved by harvest, the war would often be called off for a year.

Professional armies form when a society has grown wealthy enough to support a large number of men who do not work the fields or perform other productive work. The soldiers are usually housed in barracks and segregated from the civilian population to minimize friction, and the state provides their food, weapons, armor, and training. They are almost always more effective than the amateurs.

Modern warfare has grown monumentally complex, and soldiers kill each other with extremely complicated weapons which require substantial training for effective use. Amateur soldiers are simply not up to the task, and most states field some kind of professional force.